Nov 4 2010
Potentially harmful levels of methemoglobin—which can build up in patients receiving certain anesthetics—are detectable using a new, noninvasive monitoring technique called pulse CO-oximetry, reports a study in the November issue of Anesthesia & Analgesia, official journal of the International Anesthesia Research Society (IARS).
The new technique provides an important advantage over conventional pulse oximetry, which can't detect methemoglobin. "We now have the ability to directly measure methemoglobin in the operating room," comments Dr. Steven L. Shafer of Columbia University, Editor-in-Chief of Anesthesia & Analgesia. "prior to the development of this technology methemoglobinemia was nearly invisible in the operating room."
Pulse CO-Oximeter Detects Dysfunctional Hemoglobin
Hemoglobin is the oxygen-carrying compound in the blood; methemoglobin is a dysfunctional type of hemoglobin that doesn't transport oxygen. "Numerous drugs, including the common topical anesthetic benzocaine, are capable of forming high concentrations of methemoglobin," Dr Shafer explains. "When this occurs, it places the patient at risk of severe hypoxia due to inadequate oxygen delivery."
Methemoglobin isn't recognized by standard pulse oximetry—the familiar "finger clip" monitor, which works by shining red and near-infrared light through blood perfused tissues. Pulse CO-oximetry works differently, using several different light wavelengths to assess methemoglobin levels, along with several other measures. The "Rainbow" pulse CO-oximeter is marketed by Masimo Corp.
In the study, twelve healthy volunteers were given a chemical (sodium nitrite) that raises methemoblobin levels. Methemoglobin levels were monitored using the pulse CO-oximeter, and the results were compared with measurements made directly in blood samples. The study was performed by Drs. John R. Feiner and Philip E. Bickler of University of California at San Francisco.
The pulse CO-oximeter proved "acceptably accurate" for use in monitoring methemoglobin. At methemoglobin levels ranging from normal to high, the pulse CO-oximetry readings agreed with the levels measured in blood samples within less than one percent agreement.
The methemoglobin levels showed high agreement even in the presence of low oxygen levels. The latest generation of pulse CO-oximeter technology overcomes a previously reported problem, where the device reported falsely high methemoglobin levels at lower levels of oxygenation.
For anesthesiologists monitoring patients during surgery, the new technology helps to solve a complex problem: how to detect high levels of methemoglobin that could impair oxygen delivery. The ability to measure methemoglobin alongside oxygen saturation monitoring will help to ensure patient safety—especially during procedures using anesthetic drugs that can raise methemoglobin levels.
"The use of this technology to monitor methemoglobin also extends out of the operating room, and could be of value in the ICU, emergency room, general hospital wards, and dentist's office, as well as in patients undergoing GI procedures or bronchoscopy, since drugs that can cause methemoglobinema can be used in all these locations." Dr. Feiner adds.
Source: Columbia University